1. Field of the Invention
The invention is related to a production method of polycrystalline GaN for use as a nutrient or source material in the ammonothermal method. Design of the reactor material to control impurities while producing high yields is discussed.
2. Description of the Existing Technology
Gallium nitride (GaN) and its related group III alloys are the key material for various opto-electronic and electronic devices such as light emitting diodes (LEDs), laser diodes (LDs), microwave power transistors, and solar-blind photo detectors. Currently LEDs are widely used in cell phones, indicators, displays, and LDs are used in data storage disc drives. The majority of these devices are grown epitaxially on heterogeneous substrates, such as sapphire and silicon carbide. The heteroepitaxial growth of group III-nitride causes highly defected or even cracked films, which hinders the realization of high-end optical and electronic devices, such as high-brightness LEDs for general lighting or high-power microwave transistors.
Most of the problems inherent in heteroepitaxial growth could be avoided by instead using homoepitaxial growth. Single crystalline group III-nitride wafers can be sliced from bulk group III-nitride crystal ingots and then utilized for high-end homoepitaxial growth of optical and electronic devices. For the majority of devices, single crystalline GaN wafers are favorable because it is relatively easy to control the conductivity of the wafer and GaN wafers will provide the smallest lattice/thermal mismatch with device layers. However, the GaN wafers needed for homogenous growth are currently expensive compared to heteroepitaxial substrates. It has been difficult to grow group III-nitride crystal ingots due to their high melting point and high nitrogen vapor pressure at high temperature. Growth methods using molten Ga, such as high-pressure high-temperature synthesis (S. Porowski, MRS Internet Journal of Nitride Semiconductor, Res. 4S1, (1999), G1.3; and T. Inoue, et al., Phys. Stat. Sol. (b), 223, (2001), 15) and sodium flux (M. Aoki et al., J. Cryst. Growth, 242, (2002) 70; and T. Iwahashi, et al., J. Cryst. Growth, 253, (2003), 1), have been proposed to grow GaN crystals. Nevertheless the crystal shape grown using molten Ga is a thin platelet because molten Ga has low solubility of nitrogen and a low diffusion coefficient of nitrogen.
The ammonothermal method is a promising alternative growth method that has been used to achieve successful growth of real bulk GaN ingots (T. Hashimoto, et al., Jpn. J. Appl. Phys., 46, (2007), L889). Ammonothermal growth has the potential for growing large GaN crystal ingots because its solvent, high-pressure ammonia, has advantages as a fluid medium including high transport speed and solubility of the source materials, such as GaN polycrystals or metallic Ga.
State-of-the-art ammonothermal method (U.S. Pat. No. 6,656,615; International Application Publication Nos. WO 2007/008198; and WO 2007/117689; and U.S. Application Publication No. 2007/0234946) requires a sufficient supply of source material. While pure Ga metal can be used as a source material, it provides an uneven growth rate as the surface of the Ga nitridizes. To provide a more stable growth rate, polycrystalline GaN is desirable. One method to produce GaN polycrystals is direct nitridization of Ga with ammonia (H. Wu, et al., Phys. Stat. Sol. (c), 2 No. 7, (2005), 2074). Nevertheless, this method can only yield powder form of GaN (i.e. microcrystalline or nanocrystalline). On the other hand, GaN polycrystals obtained as a parasitic deposition during hydride vapor phase epitaxy (HVPE) show large grains with partially faceted surfaces. The parasitic GaN polycrystals in HVPE are in the suitable shape for ammonothermal nutrients; however, since HVPE are designed to minimize polycrystalline deposits and to improve epitaxial growth, the production yield of the GaN polycrystals is very low. Thus, using parasitic GaN polycrystals in HVPE as ammonothermal nutrient is not practical for mass production of bulk GaN. A method to produce large quantities of polycrystalline source material would improve the feasibility to scale the ammonothermal growth and facilitate large-scale production of high-end GaN ingots.
All references cited herein are incorporated in their entirety by this reference.